This invention generally relates to catalytic partial and full oxidation of hydrocarbons and related reduced species using catalytic membrane reactors. Reactors containing gas-impermeable, solid state membranes with an adherent catalyst layer in combination with fixed (or packed)-bed catalyst are disclosed. Membrane materials, catalyst layers and packed-bed catalysts are selected to achieve a desired selective oxidation reaction. Catalytic membrane reactions include, among others, the partial oxidation of methane or natural gas to synthesis gas.
Catalytic membrane reactors using solid state membranes for the oxidation or decomposition of various chemical species have been studied and used previously. One potentially valuable use of such reactors is in the production of synthesis gas. See, for example, Cable et al. EP patent application 90305684.4 (published Nov. 28, 1990) and Mazanec et al. U.S. Pat. No. 5,306,411. Synthesis gas, a mixture of CO and H2, is widely used as a feedstock in the chemical industry for production of bulk chemicals such as methanol and liquid fuel oxygenates. For most efficient use in the synthesis of methanol, the ratio of H2:CO in synthesis gas should be adjusted to 2:1.
In a catalytic membrane reactor that facilitates oxidation/reduction reactions, a catalytic membrane separates an oxygen-containing gas from a reactant gas which is to be oxidized. Oxygen (O2) or other oxygen-containing species (for example, NOx or SOx) are reduced at one face of the membrane to oxygen anions that are then transported across the membrane to its other face in contact with the reactant gas.
Materials for membranes in catalytic membrane reactors must be conductors of oxygen anions, and the materials must be chemically and mechanically stable at the high operating temperatures and under the harsh conditions required for reactor operation. In addition, provision must be made in the reactor for electronic conduction to maintain membrane charge neutrality. Electronic conductivity in a reactor is necessary to maintain charge neutrality permitting anion conduction through the membrane. Electron conduction can be achieved by adding an external circuit to a reactor which allows for current flow. See: U.S. Pat. Nos. 4,793,904, 4,802,958 and 4,933,054 (all of Mazanec et al.).
Electronic conductivity can also be achieved by doping oxygen-anion conducting materials with a metal ion, as illustrated by U.S. Pat. Nos. 4,791,079 and 4,827,071 (both of Hazbun), to generate dual (electrons and oxygen anions) conducting materials. The disadvantage of this approach is that the dopant metal ions can act as traps for migrating oxygen anions, inhibiting the ionic conductivity of the membrane.
Dual conducting mixtures can also be prepared by mixing an oxygen-conducting material with an electronically-conducting material to form a composite, multi-component, non-single phase material. Problems associated with this method include possible deterioration of conductivity due to reactivity between the different components of the mixture and possible mechanical instability, if the components have different thermal expansion properties.
The preferred method for obtaining electronic conductivity is to use membrane materials which inherently possess this property.
As described in U.S. patent applications (Parent and Grandparent to this case), mixed conducting metal oxides possessing the brownmillerite crystallographic structure can be used to prepare gas-impermeable ceramic membranes for use in membrane reactors for spontaneously separating oxygen from a gas, e.g., from air, on their reducing surface and mediating transfer of this oxygen as oxygen anions to the oxidation surface of the membrane where they can participate in a selected oxidative chemical process. For example, natural gas (predominantly methane) can be spontaneously converted to synthesis gas, a mixture of carbon monoxide (CO) and hydrogen (H2) which is useful as a feedstock for preparation of liquid fuels.
The reaction to form synthesis gas is a partial oxidation that is written:
CH4+O2xe2x88x92xe2x86x92CO+2H2+2exe2x88x92
FIG. 1 illustrates schematically how this reaction would occur ideally in a ceramic membrane reactor. The membrane of FIG. 1 illustrated as having a reduction catalyst on the reduction surface and a partial oxidation catalyst on the membrane oxidation surface. FIG. 1 illustrates that molecular oxygen (O2) is reduced at the reducing surface of the membrane to form oxygen anions (O2xe2x88x92) which are conducted across the membrane (due to the presence of an oxygen gradient). O2xe2x88x92 at the oxidizing surface of the membrane reacts with methane to give the partial oxidation product CO and H2 with H2:CO ratio of 2:1.
A problem that occurs with ceramic membrane reactors is that the membrane material itself can be catalytically active toward oxygen anion changing the nature of the oxygen species that are available for reaction at the membrane surface. For example, the membrane material may catalyze reoxidation of oxygen anions to molecular oxygen. The membrane then serves to deliver molecular oxygen to the oxidation zone of the reactor. The presence of molecular oxygen can significantly affect the selectivity of a given reaction. For example, reaction of methane with molecular oxygen leads to deep oxidation of methane generating CO2:
CH4+2O2xe2x86x92CO2+2H2O.
A membrane that exhibits no substantial reactivity toward oxygen anions, yet retains ionic and electronic conductivity, i.e. a membrane that is not inherently catalytically active toward oxygen, would provide for better reaction selectivity in a membrane reactor. In this case, reactivity could be determined by choice of an adherent catalyst layer on the oxidation surface of the membrane. By appropriate choice of the adherent catalyst layer a high degree of selectivity for a desired oxidation reaction should be achievable.
The use of a membrane material which has minimal catalytic activity towards oxygen separates the oxygen transport properties of the membrane from the catalytic activity. This will allow fine tuning of catalytic activity by catalyst layer choice, in particular it will allow control of the surface oxygen species allowing selection among a variety of oxygen species at the membrane surface O2xe2x88x92, O2xe2x88x92 (superoxide), O. (radical), peroxo (O22xe2x88x92), etc.
This invention provides a catalytic membrane reactor for partial or full oxidation of reduced species, particularly of hydrocarbons. The reactor comprises a gas-impermeable membrane which exhibits ion conductivity. The membrane is also provided with electronic conduction to maintain membrane charge neutrality. Electronic conduction can be provided by an external circuit or the membrane material can itself be an electronic conductor. The reactor has an oxidation zone and a reduction zone separated by the membrane which itself has an oxidation surface exposed to the oxidation zone and a reduction surface exposed to the reduction zone. The oxidation surface of the membrane is, at least in part, covered with an adherent catalyst layer. The reduction surface of the membrane is optionally provided with an oxygen reduction catalyst. The reactor is also optionally provided with a three-dimensional catalyst in the oxidation zone of the reactor in close contact with the adherent layer on the oxidation surface of the membrane.
Preferred membranes of this invention are single phase mixed ionic and electronic conducting ceramics. In this case no external electric circuit is required to maintain membrane charge neutrality. To facilitate selective oxidation, preferred membranes are those that exhibit minimal catalytic activity for oxidation of oxygen anions, e.g., are minimally active for reoxidation of oxygen anions to molecular oxygen, on transport of oxygen anion through the membrane. These membranes deliver minimal amounts of molecular oxygen to the oxidation surface of the membrane and to the oxidation zone of the reactor and minimize deep oxidation of hydrocarbons (e.g., CH4 to CO2).
Preferred membrane materials of this invention are single-phase brownmillerite materials having the stoichiometric formula:
A2xe2x88x92xAxe2x80x2xB2xe2x88x92yBxe2x80x2yO5+zxe2x80x83xe2x80x83I
where A is an alkaline earth metal ion or mixture of alkaline earth metal ions; Axe2x80x2 is a metal ion or mixture of metal ions where the metal is selected from the group consisting of metals of the lanthanide series and yttrium; B is a metal ion or mixture of metal ions wherein the metal is selected from the group consisting of 3d transition metals, and the group 13 metals; Bxe2x80x2 is a metal ion or mixture of metal ions where the metal is selected from the group consisting of the 3d transition metals, the group 13 metals, the lanthanides and yttrium; x and y are, independently of each other, numbers greater than or equal to zero and less than or equal to two and z is a member that renders the compound charge neutral. The value of z generally is greater than zero and less than 1.0, more preferably z is greater than zero and less than or equal to about 0.5, and most preferably z is greater than zero and less than or equal to 0.3. The exact value of z depends upon the valencies and stoichiometries of A, Axe2x80x2, B, and Bxe2x80x2. Preferably x is greater then zero and less than 1 and y is greater than or equal to 1 but less than 2.
The adherent catalyst layer is preferably a mixed ionic and electronic conducting layer. The catalyst of this layer is preferably chosen to facilitate efficient mediation of O2xe2x88x92 from the membrane to the chemical species to be oxidized. Alternatively the catalyst is chosen to control the nature of the oxygen species that will interact with the chemical species to be oxidized. A preferred adherent catalyst for facilitating efficient mediation of O2xe2x88x92, and thus preferred for partial oxidation of hydrocarbons, is a mixed ionic and electronic conducting ceramic having the composition:
XaRe1xe2x88x92aZbZxe2x80x21xe2x88x92bOc
where X is Ca, Sr or Ba, Re is a rare earth or lanthanide metal, including yttrium, Z is Al, Ga, In or combinations thereof and Zxe2x80x2 is Cr, Mn, Fe, Co, or combinations thereof with 0xe2x89xa6axe2x89xa61 and 0xe2x89xa6bxe2x89xa61 and c is a number, dependent upon the oxidation states of the other components, and the values of a and b, that renders the composition charge neutral.
The adherent catalyst layer can be formed by catalyst particles with deposited metal to give a mixed conducting (ion and electronic) cermet catalyst. Preferred deposited metals include Ni, Pt, Pd, Rh, Ir, Ag and combinations thereof. Metal can be deposited from about 1 wt % to about 50 wt % on the supporting catalyst. Ni deposited on a relatively basic mixed conducting support, such as LaaSr1xe2x88x92aMnO3, where 0xe2x89xa6axe2x89xa61 and particularly where a is 0.7xe2x89xa6axe2x89xa60.9, is a preferred adherent catalyst for the partial oxidation of methane to synthesis gas.
The adherent catalyst layer can also be a catalyst, such as those listed in Table 1 or 2, which promotes partial oxidation of methane to CO and H2, promotes oxidative coupling of alkanes, particularly methane to olefins, promotes the oxidative dehydrogenation of alkanes, or which promotes oxygenate production including the partial oxidation of alkanes to alcohols, aldehydes or ketones, the partial oxidation of alkenes to epoxides or the partial oxidation of alkane to anhydrides. Membranes of formula I in combination with an appropriately selected adherent catalyst are useful in catalytic reactor membranes for the listed partial oxidation reactions.
The adherent catalyst layer also provides protection for the membrane material to prevent decomposition under operating conditions.
The optional three-dimensional catalyst can be a packed- or fluidized-bed catalysts, and preferably is a packed-bed catalyst, in close contact with the adherent catalyst layer. This catalyst is selected to promote a desired oxidation reaction. The three-dimensional catalyst can, for example, comprise a metallic catalyst deposited on a support. Preferred metals include Ni, Pt, Pd, Rh, Ir, Ag, and combinations thereof. The support can be an inert oxide or a mixed metal oxide. Inert oxides include alumina. A mixed ionic and electronic conducting material can also be used as the support. The three-dimensional catalyst may be, but need not be, the same material as the adherent catalyst layer.
This invention provides reactors as described above, membranes with adherent catalyst layers and methods of oxidizing reduced species, particularly hydrocarbons, using these reactors and membranes.